The present invention relates to a process for measuring the distribution of the concentrations of constituents of a centrifuged medium, an apparatus for performing the process and the use of the apparatus.
For controlling the operation of an oil field, the petroleum industry makes use of numerical simulations in order to forecast the impact of a given operating method on the field. This forecast requires the characterization of the properties of the outflows in the field, including the evolution of the capillary pressure and the relative permeabilities of the fluids present.
These latter properties are primary focus in evaluating workable reserves or the productivity of a field, and, over several decades, different measuring means for these properties on samples have been proposed.
One of the most widely used methods is centrifuging due to its speed, ease of performance and its possible automation (J. Hagoort, 1980: "Oil Recovery by Gravity Drainage, SPE Journal", 1980, pp. 139-150; Hassler and Brunner, "Measurements of Capillary Pressure in Small Core Samples, Trans. AIME", 160, 1945, pp. 114-123).
Centrifuging can be used according to French Patent 2,556,836 for fractional separation and grain size analysis of particles suspended in a liquid, coupled with an ultrasonic thickness sensor located outside the rotor facing the centrifugal chamber.
However, generally, centrifuging only makes it possible to measure the mean saturation of the fluids in the centrifuged sample. Therefore, it is an indirect method for obtaining the capillary pressure Pc and relative permeabilities as a function of local saturations S.
The capillary pressure Pc(S) is obtained from measuring the mean saturation, under steady state conditions, at different centrifuging speeds, averaging out certain calculation approximations and certain hypotheses on the exact distribution of the fluids and the flow conditions in the sample (limit conditions, end effects, sample homogeneity, uniform displacement, etc.). The approximations and calculation methods have been progressively improved and would now appear to be satisfactory. However, the hypotheses concerning the local distribution of the fluids and flow conditions are contradictory.
The relative permeabilities kr(S) are obtained on the basis of the evolution, under non-steady state conditions, of the mean saturation and the Pc(S) on the basis of an explicit calculation or in accordance with an implicit adjustment by a numerical model simulating centrifuging known in the art.
It would appear that these relative permeabilities are highly dependent on the function Pc(S). Moreover, these very different relative permeabilities can lead to comparable evolutions of the mean saturation in a centrifuged sample, although the local distribution of the saturation evolves in a very different way.
Both for the calculation of Pc(S) and for that of kr(S), it would appear that the measurement of the mean saturation during centrifuging is not sufficiently discriminating enough.
The measurement of local saturations in the sample would appear to be a means for obtaining a direct measurement of Pc(S), a discriminating calculation of kr(S) and a verification of standard hypotheses concerning the exact distribution of the fluids in the sample.
Aiming at this improvement, Vinegar et al, (U.S. Pat. No. 4,671,102) provide means for measuring the local distribution of saturations in a centrifuged sample. Said means consists of using one or more electromagnetic sources (X-ray or other radiation) irradiate the sample, followed by analysis of the transmitted radiations. Such means would be quite suitable for determining the distribution of local saturations and variation in time during centrifuging. However, implementation of this procedure has encountered as yet unsolved technical problems, possibly be due to the overall dimensions of the source or to the dangers inherent in handling X-ray or other sources of radiation, or in the absorption of radiation by the numerous metal parts normally present in a centrifuge.